In the glue arts, the viscosity of a glue can impact the effectiveness and adherence characteristics, whether a petroleum-based glue, polymer-based glue, or natural protein-based glue. The operating viscosity limits of some natural protein glues (e.g., soybean protein glues) are very large ranging from 500 to 75,000 cP depending upon the application and on the nature of the materials to be glued. A viscosity of 500-5000 cP can be useful for gluing materials which are highly absorbing, such as paper, soft board, and dried wood aggregates. A viscosity of 5000-25,000 cP can be useful for glue used in most wood laminating purposes (e.g., both cold and hot press). A viscosity over 50,000 cP can be useful for gluing mastic consistency wood and for laminating operations. A viscosity range of about 8000-20,000 cP has been specified for no clamp cold press techniques.
The high viscosity soybean protein glues result from increased intermolecular interactions due to unfolded protein molecules. The major forces that facilitate such interactions are electrostatic and covalent disulphide bonding between protein molecules. Ionic environments have been known to weaken the electrostatic interactions between protein molecules. Thus, the viscosity of protein glues can be varied by treating with salts or by using reducing agents without affecting the final adherence strength or water resistance. Enzymatic or alkaline hydrolysis also reduces the viscosity of protein glues. Also, a protein glue composition having a higher pH can have a higher rate of hydrolysis, better adherent strength, and improved water resistance. However, an unfortunate property of protein glues having high pH is a short storage life of less than about 48 hours, which is not sufficient for a commercial glue product.
Accordingly, natural protein glues that are prepared from natural proteins (e.g., not synthetic proteins) often have a high dependence on pH for effectiveness. Typical natural protein glues (such as soy protein glue) become competitive in strength of bond with petroleum glues around a working pH of 11. Unfortunately, when soy protein glues are prepared at a pH of 11, they become useless if not applied within 24-48 hours. As such, the storage time for strongly bonding protein glues is not compatible with the storage life needed for a commercial product. Synthetic proteins can be prepared for glue compositions; however, synthetic proteins often have properties similar to natural proteins. As a result, continued improvements in protein glues are needed.